My girlfriend and I have been together for four years. Over the last two months, we have been struggling with an issue that we still cannot seem to resolve. She started a new job in September and over the course of a month, started to question her sexuality because of a guy who had been flirting with her. Some background about the extent of flirting: calling her beautiful, irresistible, and making sexual innuendos at work, despite her saying she had a girlfriend. They also met in the morning before going to their individual work stations as well. After his mom passed and he had been away from work for two weeks, he asked if they could hang out. After my girlfriend had asked me if this was okay, I agreed. That night, as I was in class, they both admitted that they liked each other, but my girlfriend told him that she was not in a place to give him a relationship. He had stopped flirting since then. Since then, my girlfriend had admitted her feelings to me about him, and has wanted an open relationship in order to explore this new side of her. She has always identified as lesbian, but is now unsure. I always believed monogamy to be a core value for me, but for someone that I love this much, I would at least like to compromise as she has asked for an open relationship. Five days ago my girlfriend texted him and asked why he keeps trying to hang out with her if she knows she can’t give him what he wants, and he’s also talking to three other girls. He had texted back that she is different, more than a friend, and nothing like these girls. She told him it was nice to see where his head is at. She hid this from me until I asked her if she was hiding anything from me. I bring this up because I thought we had gotten a little better, but in a way seeking that validation has really destroyed me. After days of talking, compromising, we decided that if she needed to explore, I would prefer it with someone she does not have feelings for. As a result, she is now on a dating app, getting to know different guys and she has stopped talking to the guy at work. She no longer meets him in the morning. Even though she reassures me that I am the one she wants to be with, I still feel sad deep down knowing I will never quite be enough in that way. She consistently reassures me that it has nothing to do with me, however I can’t help feeling this way. We have four years of history, and a year of that I had been away on military duties. We had always remained romantically faithful and despite everything going on, she is always honest. She told me that she will always want to involve other people in our sex life, but is willing to put that aside to be with me. I told her I have always been open to everything sexually, as long as it does not involve other people. She told me that will not fulfill her, and as a result, I said I would emotionally distance myself so that she could explore. Are there any alternatives to this?

TLDR; GF questioned her sexuality after four years of dating and now wants to fuck with a guy. Im very monogamous and feel terrible holding her back but also sharing her. What are some alternatives?